1. Field of the Invention
The present invention relates to a microwave oven, and more particularly, to an inverter circuit of a microwave oven driving a magnetron in an inverter system.
2. Background of the Related Art
Home appliances for cooking comprise a gas range, a gas oven range, a microwave oven and an electric oven. Among the home appliances for cooking, the microwave oven heats an object provided in a cavity with a microwave oscillated in a magnetron MGT therein. Generally, the microwave oven uses an inverter circuit to control drive of the magnetron.
An inverter circuit of a microwave oven according to the related art will be described as follows.
As shown in FIG. 1, the inverter circuit of the microwave oven according to the related art is provided with a rectifying circuit part 2 for rectifying an AC power from a power source 1 to be connected to both sides of the power source 1. Then, both output terminals of the rectifying circuit part 2 are respectively connected to a primary side coil of a high voltage transformer HVT (omission of a reference number) and a switching device IGBT 4, and a secondary side coil of the HVT is connected to a magnetron 3.
Also, an input current detecting part 5 is connected to one end of the power source 1 to convert the current outputted from the power source 1 to a voltage level and to provide the voltage level to a micom 6. The micom 6 detects the voltage level outputted from the input current detecting part 5, and controls turning on/off operations of first, second and third photo couplers PC1, PC2 and PC3 according to the detected voltage level and a preset reference voltage by a user. That is, the micom 6 turns on only first photo coupler PC1 according to the detected voltage level and the preset reference voltage, or turns on all of the first, second and third photo couplers PC1, PC2 and PC3. That is, the power is divided by the first, second and third photo couplers PC1, PC2 and PC3 to make the voltage level of the micom 6 corresponding to the voltage level of the inverter circuit.
According to operations of the first, second and third photo couplers PC1, PC2 and PC3, the voltage divided and outputted by first, second, third and fourth resistances R1, R2, R3 and R4 is inputted to a reference level output part 7. Then, the reference level output part 7 outputs a reference voltage of a predetermined DC level corresponding to the voltage outputted from the first, second, third and fourth resistances R1, R2, R3 and R4.
In a trigger circuit part 8 connected to lower and upper sides of the primary side coil of the HVT, when a voltage applied to the upper side of the primary side coil of the HVT is higher than a voltage applied to the lower side of the primary side coil of the HVT, the trigger circuit part 8 outputs a trigger signal. Also, a triangular wave output part 9 is connected to the trigger circuit part 8 to generate a triangular wave according to the trigger signal outputted from the trigger circuit part 8.
A comparing part 10 is connected to the triangular wave output part 9 and the reference level output part 7. The comparing part 10 receives the reference voltage outputted from the reference level output part 7 through a non-inversion (+) terminal, and receives the triangular wave generated in the triangular wave output part 9 through an inversion (−) terminal. At this time, if the input of the non-inversion (+) terminal is higher than the input of the inversion (−) terminal, to be in a case that the reference voltage is higher than the triangular wave, a high signal is outputted. Then, the signal outputted from the comparing part 10 is provided to the switching device IGBT through a driving part 1, to drive the switching device IGBT 4 for turning on/off operations of the magnetron 3.
The signal outputted from the comparing part 10 is controlled in turning on/off of the first, second and third photo couplers PC1, PC2 and PC3. For example, in case all the first, second and third photo couplers PC1, PC2 and PC3 are turned on, the reference voltage outputted from the reference level output part 7 is high, so that the a width (Ton) of a driving signal of the switching device 4 outputted from the comparing part 10 becomes wide ((A) of FIG. 2).
If the first and second photo couplers PC1 and PC2 are turned on, the width of the driving signal outputted from the comparing part 10 is shown as (B) of FIG. 2. Meanwhile, if only first photo coupler PC1 is turned on, the width of the driving signal outputted from the comparing part 10 is shown as (C) of FIG. 2.
However, the inverter circuit of the microwave oven according to the related art has the following disadvantages.
First, the inverter circuit of the microwave oven according to the related art requires the plurality of photo couplers as means of controlling the output, so that the unit manufacturing cost of production (microwave oven) is increased in that the photo coupler is expensive device.
Also, the micom receives the output (present current) of the input current detecting part by feedback, and controls the output of the inverter circuit, whereby a program processing time of the micom is slow, and it is difficult to control the inverter circuit correctly.